As was pointed out in the aforementioned copending application, small internal combustion engines can comprise a magneto for generating the pulse or pulses which trigger ignition in those cylinders in which compression of the fuel-air mixture has occurred.
In early designs of such engines, the electrical pulse generated by the magneto, i.e. the magnet/coil or winding assembly, which were relatively rotated by the shaft of the engine, fired the sparkplugs in appropriate order, the voltage level of the pulse being increased by the action of breaker points and a voltage multiplying coil or high voltage transformer or so-called spark coil. Firing control or proper timing of the "advance" was effected by mechanical factors such as the offsetting of various parts of the system with respect to one another.
The "advance" of the spark required for increased speed was effected by mechanically moving the element of the triggering system in such cases.
These systems eliminate the need for a battery or other source of electrical energy and hence use of magneto engines have found widespread application where such batteries and battery-charging systems are inconvenient, namely, for small engines, with outputs below, say, 100 hp, and in a wide variety of assemblies and for various uses.
In recent years, efforts have been made to develop electronic ignition systems utilizing magneto-controlled firing, substituting electronic switching to eliminate the breaker points which have caused reliability problems, especially for military and government applications where the engines must conform to extremely rigid performance specifications.
Such electronic ignition systems include those of the references in the file of the aforementioned copending application of which U.S. Pat. No. 3,941,110 is of particular interest.
In that system, the magneto winding is separate from the high voltage coil and the firing is controlled by triggering the discharge of a condenser or capacitor through the high voltage coil using a SCR as a switching element, the capacitor being charged through a rectifier. To the gate of this SCR, a transistor timing circuit with a Zener diode or a Zener diode timing circuit is provided.
In that system it is pointed out that the Zener diode eliminates automatic advance inherent in earlier SCR triggered circuits with temperature, and therefore allows the wave form in the magneto winding to be employed to trigger the SCR.
Mention may also be made of U.S. Pat. No. 4,342,304 in which a programmable unijunction transistor is employed to control the pulse through the high voltage transformer, although here no magneto arrangement is provided. Another electrical advance device for ignition timing is found in U.S. Pat. No. 3,861,372 in which, however, a separate winding is provided for the electronic circuitry controlling the advance.
Thus, while it is clear that electronic ignition systems have been developed in the past, there are still problems which were not resolved by them. In general, it can be said that in the past the electronic ignition systems which were practically available were not very reliable due to the fact of their complexity with many discrete components active and passive, relying on their parameter dispersion as evidenced by the fact that, upon the purchase of a large number of self-contained electronic ignition control systems, even when they were purported to comply with government and military specifications, it was found that a substantial proportion did not perform up to these standards, were inoperative and required replacement.
The circuits described in the above-mentioned copending application were designed to overcome these drawbacks and provide a highly efficient automatic advance control circuit which was not only simple but reliable.
In that system, the nonlinear network connected to the magneto winding and logarithmically responsive to a signal generated therein for triggering the control electrode of the SCR to discharge the condenser through the high voltage transformer to fire the spark plug provided automatic advance and retardation of the firing point depending upon the logarithmic response of the nonlinear network:
In an essential embodiment, that network consisted of:
a blocking diode connected to the magneto winding at one side of the blocking diode;
a transducer having a base connected to another side of the blocking diode and also having two principal electrodes;
a series network of a Zener diode and a resistor interposed between the aforementioned other side of the blocking diode and the base of the transistor and across the base and one of the principal electrode (emitter or collector) of the transistor; and
means including a load resistor in series with the other principal electrode for directly tapping a signal across this resistor and applying it directly to the gate or control electrode of the SCR to trigger the switch formed thereby.
This system was found to be highly reliable in practice as a self-contained unit and devoid of the drawbacks of earlier systems.